1. Field of the Invention
This invention relates to shaped articles of resin for use in various household utensils and a method for the production thereof.
2. Description of the Related Arts
In recent years, the development of resins possessing a great variety of characteristic properties has come to encourage the resins to find extensive utility in household utensils represented by household electric appliances and fixtures. Particularly, the utilization of the thermal and electrical insulating properties of these resins in articles of tableware and housings for household electric appliances has been the subject of various studies for development.
For the production of shaped articles of such resins, the method which comprises filling the cavity of a mold consisting of a male segment and a female segment with a thermally melted resin, allowing the molten resin to cool and solidify in the cavity, and thereafter removing a shaped article of resin from the mold has been generally adopted. When a shaped article of resin having a large wall is produced by this method, however, the product is heavy because of high density and liable to sustain strain in the form of so-called sink marks and warps because of shrinkage of resin.
For the solution of this problem, there have been several proposals. For example, (1) the so-called gas counter pressure method which comprises injecting a resin having incorporated therein a foaming agent into a pressurized cavity of a mold, thereafter decompressing the interior of the cavity, and foaming the resin inside the cavity as disclosed in U.S. Pat. No. 3,960,996; (2) the so-called sandwich molding method which comprises injecting a skin-layer resin containing no foaming agent, then injecting a core-layer resin incorporating therein a foaming agent, and foaming exclusively the core-layer resin; (3) a method which as disclosed in Japanese Patent Application Laid-open SHO 50(1975)-17476 comprises preliminarily keeping the interior of a cavity under pressure with compressed air, filling the cavity with a skin-layer resin and a core-layer resin both of a foaming formula by the use of an injection molding device of a desired mechanism, relieving the cavity interior of the pressure after the cavity has been filled to capacity with the resins, and foaming the skin layer and the core layer simultaneously while imparting a smooth surface to the skin layer; (4) and the method which comprises injecting a resin into the cavity of a mold and thereafter injecting a highly compressed gas into the mass of resin thereby obtaining a hollow shaped article as disclosed in Japanese Patent Publication SHO 57(1982)-14,968.
The shaped article obtained by the sandwich molding method, which foams exclusively the core-layer resin sustains strain only sparingly because the shrinkage of the skin-layer resin is internally repressed by the pressure arising from the foaming of the core layer. Hence the shaped article assumes a smooth surface excellent in rigidity and impact strength because the skin-layer resin is not foamed.
These methods have their own merits and demerits. Moreover, there has been expressed in the industry a desire to develop shaped articles of light weight. Presently, the desirability of perfecting a method for producing shaped articles enjoying ample strength, light weight, and good appearance has been finding enthusiastic recongnition.